A re-entrant cam is one in which the flanks, connecting the arcuate base and tip of the cam, have concave portions. Many designs of internal combustion engines have camshafts which carry re-entrant cams which are believed to provide superior control of the opening and closing of inlet and exhaust valves. Such camshafts are generally ground using grinding machines which have relatively small diameter grinding wheels to enable the concave portions to have relatively small radii of curvature.
However, where such a grinding wheel is mounted on a spindle assembly that extends parallel to the axis of the workpiece (on which the camshaft is to be ground), the smaller radius of the grinding wheel, in effect, limits the maximum permissible size of the spindle assembly as too large an assembly may collide with the workpiece. In an attempt to solve this problem, some known designs of grinding machine, for example as shown in U.S. Pat. No. 5,392,566, use two grinding wheels, a larger diameter wheel for obtaining the general cam profile, and a smaller diameter wheel which is subsequently applied to the workpiece to create the concave flanks. However, the need to provide two grinding wheels inevitably increases the complexity of this arrangement compared with a machine having just the one grinding wheel.
A larger spindle assembly can be used to support a small grinding wheel if the spindle assembly extends at a non-parallel angle to the angle of the workpiece. An example of such an arrangement is shown in WO 97/44159 in which a conical grinding wheel is mounted on a spindle which extends at an angle of approximately 10 to 30° to the workpiece axis. The wheel is conical, and is so arranged that the cutting line, i.e. the zone of contact between the grinding wheel and the workpiece, extends substantially parallel to the workpiece axis. To that end, it is necessary to ensure that, as the workpiece rotates, the angular position of the cutting line relative to the grinding wheel axis remains unchanged. This is achieved in WO 97/44159 by moving the grinding wheel and its spindle assembly in a linear vertical direction (perpendicular to the in-feed axis).
However, such a movement can cause uneven cutting conditions which, in turn give rise to problems of coolant application and limitations to the minimum radius of curvature that can be formed in the cam flanks.